The present invention relates to a method and device for the blow moulding of hollow bodies from thermoplastic material.
For the mass production of hollow bodies such as bottles, containers, tanks, etc. from thermoplastic material, the technique of blow moulding is almost universally used, this technique involving trapping a tubular portion of molten thermoplastic material in a suitable blow mould, shaping this parison in the mould using a blowing nozzle which is inserted into the parison and injects therein a pressurized gas, cooling the hollow body thus moulded in the mould and extracting it from the mould.
In order to obtain high productivity during such manufacturing, it is advantageous to accelerate each production step, and in particular the cooling step, which may prove time consuming in particular during the production of hollow bodies with a high wall thickness.
For this purpose, it has in particular been proposed, during the blowing or thereafter, to inject a gas held at a very low temperature (air, cryogenic nitrogen, carbon dioxide, etc.) into the hollow body produced, this cooling of the inner wall being combined with the cooling of the outer wall caused by the mould.
Moreover, for certain applications, it is advantageous to produce hollow bodies from thermoplastic material which exhibit improved impermeability to gases and to liquids.
Thus, for example, it is known that hollow bodies such as fuel tanks made from polyolefins and in particular from high-density polyethylenes, allow a small quantity of the hydrocarbon stored to diffuse through their wall, and consequently, in view of the ecological standards for controlling pollution and respecting the environment, and the conditions imposed by users, it is of paramount importance to carry out a highly efficient treatment for rendering such hollow bodies impermeable, in order to greatly reduce subsequent release of hydrocarbons.
One treatment which proves highly effective for this purpose consists in the surface treatment of the inner wall of these hollow bodies by a reactive gas (fluorine, sulphur trioxide, etc.), it being advantageous for this treatment to be carried out on line during their moulding either by using a reactive gas as the blowing gas or by injecting a reactive gas into the hollow bodies after they have been moulded using a conventional gas (air, nitrogen).
However, when cooling or surface treatment techniques are employed, the cooling or treatment gas is introduced into the moulded hollow bodies and held under pressure throughout the treatment. Consequently, when the turbulence caused by injecting the cooling or treatment gas is dissipated, that is to say, in fact, when the final pressure is reached, the heat exchanges or the treatment reaction are restricted to between the inner wall of the hollow body and the portion of the gas which is situated in direct contact therewith. The result of this is therefore that only a small proportion of the entire quantity of gas injected into the hollow body is found to be actually effective for the desired purpose.
Consequently, for example, during surface treatment of the inner wall of a hollow body such as a tank made from polyolefin intended to contain a mixed fuel containing a compound comprising oxygen, such as methanol, it is necessary to greatly extend the length of treatment with a view to obtaining finally an acceptable impermeability.